In the mining industry, run-of-the-mine (ROM) ores are crushed and pulverized to detach (or liberate) the valuable components from waste rocks. Although ROM coal is rarely crushed, a significant portion is present as fine coal. The pulverized ores and fine coal are then separated using appropriate methods. One of the most widely used methods of separation is froth flotation. In this method, a pulverized ore (or fine coal) is mixed with water to form a slurry, to which surfactants known as collectors are added to render selected constituent(s) hydrophobic. For the case of processing higher-rank coals such as bituminous and anthracite coals, which are naturally hydrophobic as mined, no collectors may be necessary. When these materials are not sufficiently hydrophobic, hydrocarbon oils are added to enhance their hydrophobicity. The hydrophobized (or naturally hydrophobic) particles are then collected by the air bubbles introduced at the bottom of a flotation cell. It is believed that the bubble-particle adhesion is driven by hydrophobic attraction. The air bubbles laden with hydrophobic particle rise to the surface of the aqueous pulp, while hydrophilic particles not collected by the air bubbles exit the cell. Thus, flotation produces two products, i.e., floated and unfloated. The more valuable of the two is referred to as concentrate, and the valueless is referred to as tailings (or refuse).
The concentrates are dewatered before they can be further processed or shipped to consumers, while the tailings (or refuse) are discarded with or without extensive dewatering. The dewatering process consists of several steps. In the first step, a slurry is thickened to 35 to 75% solids in a large settling tank, while free water is removed from the top and recycled back to the plant. In the second step, the thickened pulp is subjected to a mechanical dewatering process, such as filtration or centrifugation, to further remove the water. However, this process is inefficient, particularly when the mineral (or coal) particles are fine. In general, the moisture content in the dewatered product increases with decreasing particle size, which indicates that the residual moisture is mostly due to the surface water, i.e., the water molecules that are strongly adhering to the surface. For sulfide mineral concentrates, the filtered products contain typically 12 to 18% moisture by weight. For coal, the residual moistures are higher (20 to 30% by weight) due to its low specific gravity. Very often, these products need to be further dewatered in a third and the most costly step, i.e., thermal drying, which may be an option for high priced materials. However, it is not so for low-priced commodities such as coal. Even for the high-priced materials, elimination of the third step has significant economic and environmental advantages.
At present, the costs of cleaning and dewatering fine coal (finer than 0.5 mm) are approximately 3 times higher than those for cleaning coarser coal. For this reason, it is often more economical to discard the fines, if the fine coal constitutes only a small fraction of the product stream. This is typically the case with many coal producers around the world. In the U.S. alone, it is estimated that approximately 2 billion tons of fine coal has been discarded in abandoned ponds, while approximately 500 to 800 million tons of fine coal have been discarded in active ponds. On a yearly basis, the U.S. coal producers discard approximately 30 to 50 million tons of fine coal to ponds. This represents a loss of valuable natural resources and causes significant losses of profit to coal producers. The U.S. coal producers are blessed in that the fines fractions constitute only 5 to 20% of their product streams. In countries where coals are more friable, the fines fractions can be in the 20 to 50% range. In this case, coal producers can no longer afford to discard the fines. It is unfortunate that there are no technologies available today, other than the costly thermal drying, to lower the moisture of coal fines.
The difficulty in dewatering fine particulate materials may be explained from first principles. Those skilled in the art consider that a filter cake consists of a series of capillaries of different radii, from which water is removed during the process of vacuum or pressure filtration. The water can be removed only when the pressure drop applied across the filter cake exceeds the pressure of the water present inside the capillaries. The pressure, Δp, in the capillary of radius, r, can be calculated using the Laplace equation:                                           Δ            ⁢            p                    =                                    2              ⁢                              γ                23                            ⁢              cos              ⁢                                                           ⁢              θ                        r                          ,                            [        1        ]            in which γ23 is the surface tension at the water 3 and air 2 interface and θ is the contact angle of the inner walls of the capillary under consideration. In filtration, the capillary wall is made of the surfaces of the particles in the cake, and the effective capillary radius decreases with decreasing particle size. The contact angle is the most widely used measure of particle hydrophobicity (water-hating property). In the cessile drop method, a drop of water is placed on the surface of interest and the angle is measured through the aqueous phase. Thus, the term contact angle used in the present invention refers to the water contact angle, which increases with increasing surface hydrophobicity. Eq. [1] suggests that the capillary pressure increases with decreasing capillary radius, which explains the difficulties encountered during the filtration of fine particles. If a filter cake contains capillaries of different radii, it would be more difficult to remove the water from the finer capillaries. At a given pressure drop applied across the filter cake, one can see that the water trapped in the capillaries that are smaller than certain critical radius (rc) cannot not be removed. Thus, the moisture of a filter cake should be determined by the amount of the water trapped in the capillaries smaller than the critical capillary radius.
Eq. [1] suggests three ways of achieving low cake moistures during filtration. These include i) surface tension lowering, ii) capillary radius enlargement, and iii) contact angle increase. Various chemicals (dewatering aids) are used to control these parameters. One group of reagents is the surfactants that can lower the surface tension. Most of the dewatering aids used for this purpose are ionic surfactants with high hydrophile-lipophile balance (HLB) numbers. Sodium laurylsulfate and sodium dioctylsulfosuccinate, whose HLB numbers are 40 and 35.3, respectively, are typical examples. Sing (Filtration and Separation, March, 1977, pp. 159-163) suggested that the former is an ideal dewatering aid for coal because it does not adsorb on the surface, which in turn allows for the reagents to be fully utilized for surface tension lowering. The U.S. Pat. No. 5,346,630 teaches a method of pressure spraying a solution of a dewatering aid from a position within the filter cake forming zone of a filter just prior to the disappearance of the supernatant process water. This method, which is referred to as torpedo-spray system, ensures even distribution of the dewatering aid without becoming significantly diluted by the supernatant process water.
It should be noted here that high HLB surfactants are also used as wetting agents for hydrophobic materials such as coal. Recognizing that dewatering is essentially a de-wetting process, it is difficult to see how one type of reagents can be used for both. It is well known that high HLB surfactants adsorb on hydrophobic non-wetting surfaces with inverse orientation, i.e., with hydrocarbon tails in contact with the surface and the polar heads pointing toward the aqueous phase. Thus, high HLB surfactants can lower the surface tension, but they can also dampen the hydrophobicity and decrease the contact angle. For this reason, the high HLB surfactants used as dewatering aids can actually cause an increase in moisture content. Furthermore, the reagents remaining in filtrate eventually return to the flotation circuit and cause adverse effects.
Various polymeric flocculants are used as dewatering aids. The role of these reagents is to increase the effective size of the particles in the filter cake, so that the pore radii are enlarged. This will greatly reduce the capillary pressure and, hence, increase the filtration rate. However, most of the flocculants used as dewatering aids are hydrophilic. Therefore, their adsorption dampens the hydrophobicity of the mineral or coal concentrates that are mildly hydrophobic by virtue of collector adsorption or by nature. Furthermore, the particles form small capillaries within each floe created by organic flocculants. Therefore, the method of using polymeric flocculants for dewatering has limitations. It has been reported that flocculants are capable of reducing dewatering rate but not necessarily the final cake moisture (Meenan, Proceedings of the Industrial Practice of Fine Coal Processing, Society of Mining Engineers, pp. 223-229, 1988).
Various electrolytes can also be used to coagulate the particles to be filtered, and improve dewatering. Groppo and Parekh (Coal Preparation, 1996, vol. 17, pp. 103-116) showed that fine coal dewatering improves considerably in the presence of divalent and trivalent cations. They found this to be the case when using cationic, anionic and nonionic surfactants.
The U.S. Pat. No. 5,670,056 teaches a method of using non-ionic (or neutral) low HLB surfactants and water-soluble polymers as hydrophobizing agents that can increase the contact angle above 65° and, thereby, facilitate dewatering processes. Mono-unsaturated fatty esters, fatty esters whose HLB numbers are less than 10, and water-soluble polymethylhydrosiloxanes were used as hydrophobizing agents. The fatty esters were used with or without using butanol as a carrier solvent for the low-HLB surfactants. This invention disclosure lists a group of particulate materials that can be dewatered using these reagents. These include coals, clays, sulfide minerals, phosphates, metal oxide minerals, industrial minerals and waste materials, most of which are hydrophilic without suitable pretreatment. The use of the low HLB surfactants disclosed in the U.S. Pat. No. 5,670,056 may be able to increase the contact angles of the minerals that are already hydrophobic, but not for the hydrophilic particles.
There are several other U.S. patents, which disclosed methods of using low HLB surfactants as dewatering aids. The U.S. Pat. Nos. 4,447,344 and 4,410,431 disclosed methods of using water insoluble nonionic surfactants with their HLB numbers in the range of 6 to 12. These reagents were used together with reagents (hydrotropes) that are capable of keeping the surfactants in solution or at the air-water interface rather than at the solid-liquid interface, so that they can be fully utilized in lowering surface tension. Thus, the role of the low HLB surfactants disclosed in this invention is different from that of the surfactants disclosed in the U.S. Pat. No. 5,670,056. They do not to adsorb on the surface of the particles and enhance their hydrophobicity. The low HLB surfactants, disclosed in the U.S. Pat. Nos. 4,447,344 and 4,410,431, are the reaction products of one mole equivalent of a primary alcohol containing 6 to 13 carbons with 2 to 7 mole equivalents of ethylene oxide.
The U.S. Pat. No. 2,864,765 teaches a method of using another nonionic surfactant, a polyoxyethylene ether of a hexitol anhydride partial long chain fatty acid ester, functioning alone or as a solution in kerosene. However, the disclosure does not mention that the nonionic surfactant increases the hydrophobicity of moderately hydrophobic particles. Furthermore, the compounds disclosed are essentially not adsorbed upon the solid surface of the ore particles and remain in the filtrate, as noted in the U.S. Pat. No. 4,156,649. In the latter patent and also in the U.S. Pat. No. 4,191,655, methods of using linear or branched alkyl ethoxylated alcohols as dewatering aids were disclosed. They were used in solutions of hydrocarbon solvents but in the presence of water-soluble emulsifiers such as sodium dioctylsulfosuccinate. As has already been discussed, the use of high HLB surfactants can dampen the hydrophobicity due to inverse orientation and increase the capillary pressure.
The U.S. Pat. No. 5,048,199 disclosed a method of using a mixture of a non-ionic surfactant, a sulfosuccinate, and a defoaming agent. The U.S. Pat. No. 4,039,466 disclosed a method of using a combination of nonionic surfactant having a polyoxyalkylene group and an anionic surfactant. The U.S. Pat. No. 5,215,669 teaches a method of using water-soluble mixed hydroxyether, which is supposed to work well on both hydrophobic (coal) and hydrophilic (sewage sludge) materials. The U.S. Pat. No. 5,167,831 teaches methods of using non-ionic surfactants with HLB numbers of 10 to 14. This process is useful for dewatering Bayer process alumina trihydrate, which is hydrophilic. The U.S. Pat. No. 5,011,612 disclosed methods of using C8 to C20 fatty acids, fatty acid precursors such as esters or amides, or a fatty acid blend. Again, these reagents are designed to dewater hydrophilic alumina trihydrate.
The U.S. Pat. No. 4,206,063 teaches methods of using a polyethylene glycol ether of a linear glycol with its HLB number in the range of 10 to 15 and a linear primary alcohol ethoxylate containing 12 to 13 carbon atoms in the alkyl moiety. These reagents were used to dewater mineral concentrates in conjunction with hydrophobic alcohols containing 6 to 24 carbon atoms. The composition of this invention was preferably used in conjunction with polymeric flocculants. Similarly, the U.S. Pat. No. 4,207,186 disclosed methods of using a hydrophobic alcohol and a non-ionic surfactant whose HLB number is in the range of 10 to 15.
It is well known that oils can enhance the hydrophobicity of coal, which is the reason that various mineral oils are used as collectors for coal flotation. The U.S. Pat. No. 4,210,531 teaches a method of dewatering mineral concentrates using a polymeric flocculant, followed by a combination of an anionic surfactant and a water-insoluble organic liquid. The use of flocculant and ionic surfactants may be beneficial in dewatering, but they could dampen the hydrophobicity of the particles and, hence, adversely affect the process. The U.S. Pat. No. 5,256,169 teaches a method to treat a slurry of fine coal with an emulsifiable oil in combination with an elastomeric polymer and an anionic and nonionic surfactant, dewatering the slurry and drying the filter cake, where the oil reduces the dissemination of fugitive dusts. The U.S. Pat. No. 5,405,554 teaches a method of dewatering municipal sludges, which are not hydrophobic, using water-in-oil emulsions stabilized by cationic polymers. The U.S. Pat. No. 5,379,902 disclosed a method of using heavy oils in conjunction with two different types of surfactants, floating the coal-emulsion mixture, dewatering the flotation product and drying them for reconstitution. The U.S. Pat. No. 4,969,928 also teaches a method of using heavy oils for dewatering and reconstitution.
The U.S. Pat. No. 4,770,766 disclosed methods of increasing the hydrophobicity of oxidized and low-rank coals using additives during oil agglomeration. The main objective this process is to improve the kinetics of agglomeration and ultimately the separation of hydrophilic mineral matter from coal. The additives disclosed in this invention include a variety of heavy oils and vegetable oils, alcohols containing 6 or more carbon atom, long-chain fatty acids, etc. When these additives were used, the product moisture was lower than would otherwise be the case. However, the process requires up 300 lb/ton of additives and uses 45 to 55% by volume of an agglomerant, which is selected from butane, hexane, pentane and heptane.
The U.S. Pat. No. 5,458,786 disclosed a method of dewatering fine coal by displacing water from the surface with a large amount of liquid butane. The spent butane is recovered and recycled. The U.S. Pat. No. 5,587,786 teaches methods of using liquid butane and other hydrophobic liquids for dewatering other hydrophobic particles.